The invention pertains to key telephone systems of the type that use the tip and ring conductors of a telephone line for transmitting a hold control signal between stations.
Key telephone systems (KTS) that operate exclusively over the standard tip and ring conductors of the service phone lines available from a central office (or PBX/ESS) have a number of advantages over earlier systems that require auxiliary wiring between each KTS station. In the former, the tip and ring conductors are used for transmitting a hold condition control signal between the various KTS stations in the manner more fully disclosed in U.S. Pat. No. 4,132,860 for "HOLD CONTROL FOR KEY TELEPHONE SYSTEM" by Harry R. Rasmussen, and U.S. Pat. No. 4,133,985 for "KEY TELEPHONE SYSTEM" by Harry R. Rasmussen, Gene A. Kimzey and Robert D. Hailey. Systems such as disclosed in the above patents can be manufactured economically, and installed readily, and at a significant cost savings because of the elimination of the usual interstation, auxiliary wiring, and because of the elimination of a remotely mounted central control unit, required in the prior, standard KTS installations. A KTS using the tip and ring conductors exclusively, as disclosed in the above-mentioned patents, may be made up of a plurality of small compact control units, one for each telephone station, that may be individually mounted to the case of an associated telephone set as disclosed in U.S. Pat. No. 4,061,888 for "CONTROL UNIT MOUNTING AND INTERCONNECTING APPARATUS FOR TELEPHONE SETS" by Harry R. Rasmussen.
While KTS installations of the type disclosed in the above-mentioned patents provide a number of important advantages, such systems do not include the intercom capability that normally accompanies the prior, standard KTS equipment. In the latter case, the interstation, auxiliary wiring provides an available transmission path for intercom signals. Thus, for example, a receptionist who answers an incoming call at a first station, can place that caller on hold, and then use the intercom to alert a person at another station of the existence of a call on hold. In contrast, the KTS units that employ the tip and ring conductors as the exclusive means of transmitting interstation coordinating signals, such as the hold signal, do not have the interstation, auxiliary wiring available for intercom purposes. Thus, while busy and hold conditions may be visually indicated by line condition indicator lamps provided at each station, the absence of intercom transmission over auxiliary wiring makes it difficult for one station to alert another station to the presence of a call received or placed by the former and intended for the latter.
Accordingly, it is an object of the present invention to provide for interstation signalling, during a call on hold condition, in a KTS of the type in which all interstation coordinating signals are transmitted over the tip and ring conductors of the telephone service lines.
A more specific object is to provide in such a KTS for enabling a person at a first of the KTS stations to activate a signalling device, such as a buzzer, located at a second station so that a person at the second station can be alerted to the presence of a call on one of the available KTS lines.
Another more specific object of the invention is to provide for such interstation signalling by low cost, easily manufactured modifications to the hold control circuitry that is already available in a KTS of the characterized type.